Electrophotographic imaging processes and techniques have been extensively described in patents and other literature. The initial image forming step in the electrophotographic process cycle is the creation of an electrostatic latent image on the surface of a photoconductive element. This can be accomplished by charging the photoconductive element in the dark, such as through use of a corona or biased roller charging element. An electrostatic latent image is then formed by image-wise exposing the photoconductive element either optically or by electronic means, such as a laser or an array of light-emitting diodes. The image exposure creates free electron-hole pairs which migrate through the photoconductive element under the influence of the electric field. In such a manner, the surface charge is dissipated in the exposed regions, thus creating an electrostatic charge pattern. A visible image is then formed by depositing electrophotographic toner, comprised of electrically charged marking particles, onto the electrostatic latent image during a development step.
Two methods of development are used in electrophotography: discharged area development (DAD) and charged area development (CAD). The former uses toner of the same polarity as the surface charge on the photoconductive element. The latter utilizes toner of polarity opposite to the polarity of the charge on the element. CAD is widely used in optical copiers, while DAD is more desirable for digital applications.
The image formed in the development step is transferred to a suitable receiver, such as transparency stock or paper. It can be transferred to a receiver directly, with the assistance of, for example, an electric field or through the application of heat, pressure, or heat and pressure. Alternatively, the image can be first transferred to an intermediate member and, subsequently, transferred to the receiver. Color images can be made by transferring images comprised of the primary colors, (e.g. cyan, magenta, yellow, and black) in register, to either the receiver or the intermediate member.
After transfer, the image is permanently fused to the receiver via a suitable fusing process. In preparation for the next electrophotographic cycle, the photoconductive element is typically cleaned of residual toner because the transfer step is not 100% efficient. Cleaning efficiency is increased by electrostatically conditioning the photoconductive element and residual toner with a charging element known as a pre-clean charger. Cleaning by any of a number of methods known to one skilled in the art is then performed.
Photoconductive elements, also called photoreceptors, are composed of an electrically conductive base and at least one active layer which is insulating in the dark but which becomes conductive upon exposure to light. The base may be in one of many forms, for example, a drum, a web or belt, or a plate. The photoreceptor can comprise one or multiple active layers. The active layer(s) typically contains one or more materials capable of the photogeneration of charge carriers (electrons or holes) and one or more materials capable of transport of the generated charge carriers.
Numerous materials have been described as being useful components of the photoconductive element. These include inorganic substances, such as selenium and zinc oxide, and organic compounds, both monomeric and polymeric, such as arylamines, arylmethanes, carbazoles, pyrroles, phthalocyanines, dye-polymer aggregates, and the like. Organic compounds are particularly useful for several reasons. They can be prepared as flexible layers; as such, the copier or printer architecture is not limited to a particular configuration. Organic compounds have spectral sensitivities that can extend throughout the visible and into the near infrared regions of the spectrum. Organic compounds are amenable to low cost large area manufacturing processes. Elements prepared from organic materials are known as organic photoconductors (OPCs).
OPCs can be prepared with single or multiple active layers. In most OPCs, charge transport occurs through movement of a single type of charge carrier, electrons or holes, but not both. When only one carrier is mobile, trapped carriers of opposite sign can be created, resulting in a change in sensitometry of the active layer and in a phenomenon known as latent image hysteresis. One solution to the problem of latent image hysteresis is to separate the charge generation and transport functions into separate layers, referred to as the charge generation (CGL) and charge transport (CTL) layers, to form a dual or multi-layer photoconductive element.
A problem associated with OPCs is that the lifetime of these elements is less than desired. Physical damage to the photoconductive element incurred during the electrophotographic process, from installation or other service procedures, or from foreign objects falling into the electrophotographic engine during normal use, can significantly reduce the lifetime of the element and will impart defects in the images produced. Such defects occur at random time intervals and cannot be treated at normal service intervals.
In order to address the issue of damage to the photoconductive element, protective layers such as sol-gel overcoats are often coated onto the photoconductive element. However, in order to be effective, the charge transport properties of such overcoats must be strictly controlled. If the material is too electrically insulating, it will not permit the photoconductive element to photodischarge. This will result in poor electrostatic latent image formation. Alternatively, if the layer is too conducting, the electrical charges forming the electrostatic latent image will spread prior to development. This effect, referred to as latent image spread (LIS), will result in a loss of resolution and blurring of the image. It is particularly problematic with high quality electrophotographic engines producing latent images requiring a resolution of 600 dpi or greater. For commonly used materials such as sol-gels, the electrical conductivity is generally controlled by the addition of ionic charge conducting agents to the sol-gel formulation. However, the resistivity of such materials is highly sensitive to the humidity and can be too resistive under some conditions and too conductive under others.
A further challenge in the design of protective layers is to maintain their flexibility when used on flexible substrates, such as photoconductor elements in a belt configuration. Belts are frequently used in high speed electrophotographic processes. The belts must frequently be bent around rollers or other elements that have a small radius. Thus, the photoconductive element must be able to withstand bending repeatedly over a small bending radius. A thick protective layer will crack or peel away from the photoconductive layers under these circumstances.
Further, sol-gel overcoats are limited in the amount of protection from physical damage that they can impart to a photoconductive element. The use of sol-gel layers on flexible substrates, such as photoconductive elements in a belt configuration, puts a particular limitation on the amount of abrasion resistance that a sol-gel coating can impart. The belts must frequently be bent around rollers or other elements that have a small radius. Thus, the photoconductive element must be able to withstand bending repeatedly over a small bending radius. A thick protective layer will crack or peel away from the photoconductive layers under these circumstances so that increasing the thickness of the sol-gel to increase its protective abilities is not a solution. Increasing the hardness of the sol-gel might improve abrasion resistance, but then the coating would not be flexible and would be subject to cracking and flaking from the layers it is designed to protect. Thus, it is not clear how the abrasion resistance of these materials could be improved or how the LIS problem could be eliminated.
Another type of protective overcoat used with OPCs is diamond-like carbon (DLC). Hotomi et al., in U.S. Pat. No. 4,965,156, discloses the use of two protective layers on an organic photoconductive element. The first layer is an amorphous carbon layer which includes more than 5 atomic percent fluorine. The second, outermost layer is a similar material except that the fluorine content must be lower than 5 atomic percent. Layer thicknesses disclosed are 0.01 to 4.0 .mu.m for the first layer and 10 to about 400 angstroms (0.001 to about 0.04 .mu.m) for the second layer. Hotomi et al. teach that if the fluorine content is above 5 atomic percent in the outermost layer, it causes image fogging. Fogging can be detected by measurements of latent image spread.
U.S. Pat. No. 5,525,447 to Ikuno et al. discloses an electrophotographic photoconductor with a surface protective layer formed on the photoconductive layer. The surface protective layer is a multi-layer or graduated layer structure having at least one additive element selected from the group consisting of nitrogen, fluorine, boron, phosphorous, chlorine, bromine, and iodine. The additive element is at a higher concentration near the surface of the protective layer than at the interface between the protective layer and the photoconductive layer. When the additive element is fluorine, the fluorine to carbon atomic ratio (F/C) of 0.001 or less in the vicinity of the photoconductive layer adjacent to the protective layer and of 0.005 or more in the vicinity of the top surface of the protective layer. The layer structure is used to improve adhesion of the protective layer to the photoconductive layer. It is disclosed that adhesion is poor if the multilayer or graduated layer structure is not used. Only single layer and dual layer photoconductive elements are disclosed. Thicknesses of the protective layers in the range of 0.5 to 5 .mu.m are disclosed.
U.S. Pat. No. 4,882,256 to Osawa et al. discloses the use of a hydrogen-containing amorphous carbon overcoat layer containing one or more atoms selected from the group consisting of halogen, oxygen, and nitrogen. Oxygen concentrations in the overcoat layer of 0.1-3% are disclosed. Fluorine concentrations in the overcoat layer of 0.1-23% are disclosed. Single and dual layer photoconductive elements are disclosed.
U.S. patent application Ser. No. 08/639,374 to Visser et al. discloses the use of fluorinated diamond-like carbon outermost layers on organic photoconductive elements comprising charge transport layers containing arylamine. Fluorine concentrations of 25-65 atomic percent are claimed. Outermost layers of 0.05 to 0.5 .mu.m thickness are claimed.
Each of the inventions discussed above has a limitation in the amount of protection that it can impart to an OPC. Thicker DLC layers cannot be used because the thicker coatings tend to flake off from the active layer of the OPC. Harder DLC layers cannot be used because the method by which increased hardness is achieved also results in increased stress in the layers; the increased stress results in flaking of the DLC layer from the active layer. Further, as taught by Hotomi et al. in U.S. Pat. No. 4,965,156, there are limitations in the type of DLC layers that may be used with OPCs to avoid getting LIS. Thus, it is not clear how to increase the protective ability of DLC layers. For long process lifetimes that are becoming a necessity for photoconductive elements in modern electrophotographic processes, however, increasing the level of protection for the active layers of an OPC is crucial.
Thus, there is a need to provide a protective overcoat which has improved protective properties and which reduces the amount of LIS seen with other protective coatings, such as sol-gels.